In the congressional budget fight that shut down the federal government for 16 days and threatened default, the archenemy of many Senate Republicans was not any Democrat.

It was, by many accounts, the Senate Conservatives Fund, an outside group whose stated mission is to elect the most conservative Republican candidates to the Senate, but which has lately taken a uniquely aggressive stand against sitting Republican senators who eschew hard-line positions.

The group has used significantly more resources to attack those Republicans, including Senate Minority Leader Mitch McConnell, R-Ky., than it has spent to take on Democrats — and the dynamic has left many Senate Republicans profoundly frustrated.

“Colleagues talk about it all the time,” said Sen. John McCain, R-Ariz. “Obviously, they’re not very happy about it.”

Now, having focused for the past few months on issue advocacy — opposing legalization or "amnesty" for illegal immigrants and defunding Obamacare — the Senate Conservatives Fund is refocusing on elections with a rush of endorsements in recent days, including one for McConnell’s primary opponent, Matt Bevin.

It is the start of a new era for the Senate Conservatives Fund.

After the 2006 midterm election cycle, then-Sen. Jim DeMint, R-S.C., sensed an untapped demand for a political group independent of the Republican Party to support only conservative Republican candidates, and established the organization in 2008 to seek out and boost candidates of his ideological persuasion.

DeMint didn’t immediately plan to take on Republican incumbents, but he didn’t rule it out, either. Ultimately, however, he focused his attention on backing conservative Republicans in races for open seats or against sitting Democrats. The group has endured a number of high-profile failures, including Sharron Angle in Nevada and Christine O’Donnell in Delaware. It's also had banner successes, helping elect conservative stalwarts like Sens. Mike Lee, R-Utah, and Rand Paul, R-Ky.

“Jim [DeMint] was really good about making sure that he didn’t take on his fellow colleagues in the Senate, but when there was an opening, he went out and tried to recruit and support candidates like (Sens.) Ted Cruz and Marco Rubio,” said Sen. Lindsey Graham, R-S.C., a longtime colleague of DeMint’s. “But it seems the Senate Conservatives Fund is going in a bit of a different direction since he left.”

When DeMint retired from the Senate this year to run the Heritage Foundation, the game changed for the Senate Conservatives Fund — because not having a sitting senator in charge means never having to say you’re sorry.

“DeMint is no shrinking violet, but he probably wasn’t wild about trooping down to the Senate floor and confronting colleagues he was working to unseat. For him at the time, I imagine it was just a bridge too far,” said one conservative Republican operative. “But with DeMint no longer in the Senate, it’s freed them up.”

The group is now run by Matt Hoskins, DeMint’s former legislative director, who is trying to grow DeMint’s vision of a lean-staffed, largely virtual operation in the tech-savvy mold of President Obama’s presidential campaigns into a top-tier political group.

For a group on the rise, there could have been no better attention-getter than the fight to defund Obamacare that was spearheaded by Lee and Cruz and led to a government shutdown. The Senate Conservatives Fund demanded that Republicans follow the example of Cruz and Lee, and the two men, who maintain close ties to the group, starred in ads to help the Senate Conservatives Fund raise money.

That publicity, coupled with hundreds of thousands of dollars in telemarketing, raised roughly $3.5 million for the organization in August and September alone.

Much of that money has been used to threaten incumbent Republican senators with a primary challenge unless they embrace the group's hard line, something that has roiled the GOP's more moderate wing.

“They’re acting like bullies now,” said one Republican strategist. “They’re raising money by misleading conservatives, and they’re taking that money and threatening members of Congress.”

If anyone is surprised by how the Senate Conservatives Fund has used its war chest, however, Hoskins doesn’t understand why.

“I hear people complain that we raise money for conservative candidates. No kidding!” Hoskins told the Washington Examiner. “We’re the Senate Conservatives Fund. It’s in our name. That’s what we do, and we do it well.”

And it is only the beginning.

“In 10 years, I hope there’s 10 or 15 more Ted Cruzes and Mike Lees in the Senate,” Hoskins said.

To get there, the Senate Conservatives Fund this year looked across the Capitol to make its first endorsement in a House race — not because it plans to expand its focus from the Senate, but to begin building a farm team of potential Senate candidates, starting with Rep. Jim Bridenstine, R-Okla.

“We haven’t been shy about saying we would love to see Jim Bridenstine run for the U.S. Senate,” Hoskins said, pointing to the seat that will open in 2016 when Sen. Tom Coburn, R-Okla., retires.

Bridenstine, a freshman, won his seat in a reliably Republican district by coming out of nowhere to beat an incumbent. He told the Examiner that he had never heard of the Senate Conservatives Fund before the group approached him about a possible endorsement — but he’s warmed to their support.

“It’s actually pretty nice,” Bridenstine said, “because we’ll have a day where $20,000 shows up in the mailbox and we’re like, 'Oh, that’s a good day, what happened?' ”

The Senate Conservatives Fund asked Bridenstine to recommend other House members for potential future endorsements. None have been announced yet.

The candidates the Senate Conservatives Fund is backing and the farm team of conservatives it is building is not part of any effort to win back a Republican majority in the Senate — a top priority of the party itself. Instead, it's about ideological purity, at any cost.

“If a Republican majority is led by people who won’t defund Obamacare,” Hoskins said, “what’s the point of that?”